Most of secondary batteries, which are capable of being charged and thereby repeatedly used, are processed to be in the form of a battery pack and provided to users. In order to secure the safety of users and electronic devices, particularly in a lithium ion secondary battery having high weight energy density, some protection circuits for overcharge protection, overdischarge protection, and the like are generally built in a battery pack, and said battery has a function capable of interrupting the output of the battery pack in a predetermined case.
Using an FET switch built in the battery pack, the output thereof is turned on and off, whereby an overcharge protection operation or an overdischarge protection operation of the battery pack is performed. However, in the case where the FET switch is short-circuited and broken due to some reason; in the case where application of a lightning surge or the like causes a high current to instantly flow; or in the case where an output voltage extraordinarily decreases due to the life of a battery cell, or, on the contrary, in the case where excessive voltage is outputted, the battery pack and the electronic device must be protected from accidents, such as fire accidents. Therefore, in order to safely interrupt the output of a battery cell in any thus-postulated abnormal situation, a protection element comprising a fuse element has a function to interrupt a current path using an external signal.
As disclosed in Patent Literature 1, for such protection element of a protection circuit for lithium ion secondary batteries and the like, there has been generally employed a structure which has a heating body inside a protection element and a fusible conductor on a current path using this heating body.